Method of controlling the thickness of the lateral ledges in a cell for the electrolytic recovery of aluminum

ABSTRACT

The thickness of the lateral ledges of frozen electrolyte in a cell for the electrolytic recovery of aluminum is controlled by using the changes of level of the anode beam.

United States Patent n91 Chaudhuri 1 Aug. 19, 1975 METHOD OF CONTROLLING THE THICKNESS OF THE LATERAL LEDGES IN A CELL FOR THE ELECTROLYTIC RECOVERY OF ALUMINUM [75] Inventor: Kiranendu B. Chaudhuri, Gampel,

Switzerland [73] Assignee: Swiss Aluminum Ltd., Chippis,

Switzerland [22] Filed: Feb. 15, 1974 21 Appl. No.: 443,099

-[30] Foreign Application Priority Data Jan. 30, 1974 Switzerland 1243/74 [52] US. Cl. 204/67; 204/225; 204/245 {51] Int. Cl. CZSC 3/06 [58] Field of Search 204/67, 243 R247,

[56] References Cited UNITED STATES PATENTS 1,961,893 ,6/1934 Wadman et a1 204/245 X 3,812,024 5/1974 Goodnow et al. 204/67 FOREIGN PATENTS OR APPLICATIONS 305,202 8/1971 U.S.S.R 204/225 Primary Examiner.lohn H. Mack Assistant Examiner-Aaron Weisstuch Attorney, Agent, or Firm-Ernest F. Marmorek 5 7] ABSTRACT The thickness of the lateral ledges of frozen electrolyte in a cell for the electrolytic recovery of aluminum is controlled by using the changes of level of the anode beam.

8 Claims, 1 Drawing Figure PATENTEB AUG T 9 I975 METHOD OF CONTROLLING THE THICKNESS OF THE LATERAL LEDGES TN A CELL FOR THE ELECTROLYTIC RECOVERY OF ALUMINUM BACKGROUND OF THE INVENTION For the recovery of aluminum by electrolysis of aluminum oxide (A1 alumina) the latter is dissolved in a fluoride melt, which consists in the greatest part of cryolite Na AlF This melt is contained in a cell, the inner walls of which consist of amorphous carbon. Anodes of amorphous carbon dip from above into the melt. The aluminum separated at the cathode collects in liquid state on the bottom of the cell beneath the fluoride melt. Oxygen is released at the anodes by the electrolytic decomposition of the aluminum oxide, and combines with the carbon of the anodes to CO and C0 The electrolysis takes place in a temperature range of about 940 to 975C.

The principle of an aluminum electrolysis cell with prebaked anodes appears from the FIGURE, which shows a schematic vertical section in the longitudinal direction through part of an electrolysis cell. The steel shell 12, which is lined with a thermal insulation 13 of heat-resisting, heatinsulating material, e.g. chamotte, and with carbon 11, contains the fluoride melt (the electrolyte). The aluminum l4 separated at the cathode lies on the carbon bottom 15 of the cell. The surface 16 of the liquid aluminum constitutes the cathode. In the carbon lining 11 there are inserted iron cathode bars 17 (in this case transverse to the longitudinal direction of the cell), which conduct the electrical direct current from the carbon lining ll of the cell laterally outwards. Anodes 18 of amorphous carbon dip from above into the fluoride melt l0, and supply the direct current to the electrolyte. They are firmly connected via conductor rods 19 and by clamps 20 with the anode beam 21. The anode beam can consist of one or more conducting bars.

The current flows from the cathode bars 17 of one cell to the anode beam 21 of the following cell through conventional bus bars, not shown. From the anode beam 21 it flows through the conductor rods 19, the anodes 18, the electrolyte 10, the liquid aluminum l4, and the carbon lining 11 to the cathode bars 17. The electrolyte 10 is covered with a crust 22 of solidified melt (frozen electrolyte) and a layer of aluminum oxide 23 lying above it. Cavities 25 occur in operation between electrolyte 10 and the solidified crust 22. Against the side walls of the carbon lining 11 there likewise forms a crust of solid (frozen) electrolyte in the form of the lateral ledges 24. The thickness of the ledges 24 determines the horizontal extent of the bath of fluid aluminum 14 and electrolyte 10. With rising temperature, the thickness of the ledges 24 generally decreases. with falling temperature generally increases.

The average distance d from the lower faces 26 of the anodes to the upper surface 16 of the liquid aluminum, which is also known as the interpolar distance, can be adjusted by lifting or lowering the anode beam 21 with the help of the lifting mechanisms 27, which are mounted on pillars 28. This operates on all the anodes. Each anode can however be adjusted by raising or lowering singly. if the respective clamp 20 is opened, the conductor rod 19 is shifted relatively to the anode beam 21 and finally the clamp 20 is again closed. Because of the attack by the oxygen released during electrolysis, the anodes are consumed continuously on their lower face by about 1.5 to 2 cms per day (anode burning) according to the type of cell, and simultaneously the level of the liquid aluminum rises by about the same amount because of the separation of aluminum at the cathode.

When an anode is used up, it must be exchanged for a new one. The cell is so operated in practice that, some days after starting up, the anodes of the cell no longer have the same degree of consumption and therefore after use for several weeks they must be exchanged separately. For this reason one finds anodes of different starting age operating together, as appears from the FIGURE.

The horizontal surface, which contains the totality of the lower faces of the anodes of a cell, is known as the anode table.

The principle of an aluminum electrolysis cell with selfbaking anodes (Soederberg anodes) is the same as that of an aluminum electrolysis cell with pre-baked anodes. Instead of pre-baked anodes, anodes are used which, during the electrolytic operation, are continually baked from a green electrode paste in a steel jacket by the heat of the cell. The direct current is supplied by lateral steel rods or from above by vertical steel rods. These anodes are renewed as required by pouring green electrode paste into the steel jacket.

By breaking in the upper electrolyte crust 22 (the crusted bath surface) the aluminum oxide 23 which is above it is brought into the electrolyte 10. This operation is known as servicing of the cell. In the course of the electrolysis the electrolyte becomes depleted in aluminum oxide. At a lower concentration of for example I to 2.5% of aluminum oxide in the electrolyte, there arises the anode effect, which results in a sudden increase in voltage from the normal 4 to 4.5 volts for example to 20 volts-and above. Then at the latest the crust must be broken in and the Al O; concentration be raised by addition of new aluminum oxide.

In normal operation the cell is usually serviced periodically, even if no anode effect occurs. This cell servicing will be referred to in what follows as normal cell servicing. It occurs for example every 2 to 6 hours. In addition, as stated above, upon every anode effect the crust of the bath must be broken in and the A1 0 concentration raised by addition of fresh A1 0 which corresponds to a cell service. Thus in operation the anode effect is always associated with a cell service, which, in contrast to normal cell service, can be referred to as anode effect service.

The aluminum 14 produced electrolytically, which collects on the carbon bottom of the cell, is generally tapped once a day from the cell, e.g. by conventional sucking devices. Generally the level of the liquid aluminum 14 is brought back to an optimum value for each type of cell. This value corresponds to the desired metal level, which. can be the starting level.

An important characteristic value in the operation of a cell is its electrical base voltage. This is established empirically for each cell having regard to its age, the condition of the carbon lining 11, the composition of the electrolyte melt 10 as well the cell current intensity and current density. For the establishment of the base voltage regard is also had to the horizontal extent of the cathode surface 16, which is influenced by the thickness of the lateral ledges 24.

From the base voltage the base resistance of the cell can be calculated according to the following equation:

J R,, is the ohmic base resistance in ohms, U,, the base voltage in volts, l.65 the back electromotive force in volts and J the instantaneous cell current intensity im amps.

For the actual voltage to equal the base voltage, the interpolar distance must have an optimum value. If the cell is so operated that the horizontal extent of the cathode surface 16 remains unchanged, then generally the rise in level of the liquid aluminum above the carbon bottom is equal to the burning away of the anodes at their lower faces. The cell is designed so that these conditions are reached.

In this case the positions (levels) of the anode beam, for instance immediately after a tapping operation and immediately before the next tapping operation, will be the same.

In practice the actual interpolar distance is from time to time, e.g., between two tapping operations, larger or smaller than the optimum interpolar distance. The departures are substantially caused by irregular rise in the level of the liquid aluminum above the carbon bottom, by irregular burning away of the anodes at their lower faces, and'by variation in the horizontal extent of the cathode surface 16 as a consequence of alteration of the thickness of the lateral ledges 24. In this case, on the contrary, the levels of the anode beam, for instance immediately after a tapping operation and immediately before the next tapping operation will be different.

SUMMARY OF THE INVENTION For short time periods, e.g., between two tapping operations, only minor and negligible irregularities of burning away of the anodes occur. Therefore the differences of the levels of the anode beam are related according to my invention to the changes of the thickness of the lateral ledges of frozen (solid) electrolyse in the cell, and I use this relationship for bringing back the thickness of the lateral ledges to the desired value by carrying out procedures known to the expert in the art.

The method according to my invention for controlling the thickness of the lateral ledges of frozen electrolyte in a cell for recovery of aluminum by electrolysis of aluminum oxide dissolved in a fluoride melt comprises the following operational steps:

a. at regular time intervals during which the cell has no anode effects and during which no working operations are carried out or are still influencing the ohmic cell resistance the instantaneous ohmic cell resistance is calculated, the instantaneous values over a certain period of time are smoothed and the difference AR between this smoothed cell resistance and the base resis tance established for each cell is calculated;

b. as soon as the difference AR exceeds a limiting value given for each cell, the anode beam is raised or lowered in order to match the existing ohmic resistance with the ohmic base resistance of the cell;

c. the level of the anode beam is read by means ofa level indicator (in German Weggeber) and is stored;

d. at the latest, after one day the operational steps (a) to (c) are repeated;

e. from the two levels of the 'tlllull, beam determined by the operational steps (c) and (d), the difference AB is calculated; for which difference changes of level of the anode beam position due to tapping or addition of metal must be taken into consideration;

f. depending of the sign and magnitude of AB, known procedures are carried out which are apt to bring back the thickness of the lateral ledges of frozen electrolyte to the desired value.

A working operation mentioned in operational step (a) can be a normal servicing of the cell, an anode effect servicing of the cell, a change of anodes or a tapping of metal. Such a working operation can have a disturbing influence on the determination of the ohmic cell resistance until about one hour after the end of the working operation. In practice it is sufficient to wait half an hour after the end of a working operation before determing the ohmic cell resistances. The regular time intervals mentioned under (a) can lie between 2 seconds and 5 minutes. In practice time intervals of 10 seconds to 1 minute have proved to be advantageous.

The periods of time likeweise mentioned under (a) can lie between 1 minute and 1 hour. In practice advantageously periods of 10 minutes are chosen.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following an advantageous example of the method according to the invention is described.

At regular time intervals, for instance every 10 to 60 seconds, during which the cell has no anode effects and during which no working operations are carried out or are still influencing the ohmic cell resistance, the instantaneous cell voltage U and the cell direct current intensity J are sampled by a computer and the instantaneous ohmic cell resistance is calculated by a computer according to the equation inxl R,-,, is the instantaneous ohmic cell resistance in ohms, U the instantaneous cell voltage in volts, 1.65 the back electromotive force in volts and J the cell direct current intensity in amps.

These values of R,-,, are smoothed by a computer over a predetermined period of time, for instance over 10 to 15 minutes, and are compared, for instance every 10 to 15 minutes, with the ohmic base resistance R, of the cell. If the computer notices a difference AR between the smoothed value and the ohmic base resistance R,, and if this difference exceeds a limiting value previously given to the computer and stored in it, eg 0.5 microhms, then an order is issued by the computer in accordance with which the anode beam is raised or lowered until the instantaneous ohmic resistance of the cell is substantially equal to the ohmic base resistance of the cell. By this operation the optimum interpolar distance of the cell is reached.

Now for the first time the level of the anode beam is read by a computer with the help of a level indicator mounted on the anode beam itself. As a level indicator, a potentiometer is advantageously used. The level value is stored by the computer.

At the latest, after one day all the above mentioned operational steps to reach the optimum interpolar distance are repeated by the computer, after which the level of the anode beam is read and stored by a computer for the second time.

From the'two determinations of the levelv ofthe anode beam the computer calculates thedifferenbe AB. If between these two determinations of the levels of reached resulting in better current efficiency and lower the anode beam metal has been tapped from the cell,

If the difference AB is zero (AB 0), it is supposed that between the two determinations of the levels of the anode beam the thickness of the lateral ledges of frozen electrolyte has not changed.

If the difference is positive, e.g. AB +10 mm, the

thickness of the lateral ledges is supposed to have increased. If on the contrary the difference is negative, e.g. AB lO mm, the thickness of the lateral ledges is supposed to have decreased.

If for AB a positive or negative value has been found according to the method of the invention, one or several measures known to the experts in the art have to be carried out to bring back the thickness of the lateral ledges of frozen electrolyte to the desired value.

The most important known measures are the following:

1. Changing the electric power of the cell by changing the interpolar distance and thus changing the ohmic base resistance at constant direct current intensity of the cell. Increasing the electric power of the cell leads to a decrease of the thickness of the lateral ledges of frozen electrolyte, and vice versa.

2. Changing the electric power of the cell by changing the direct current intensity of the cell at constant interpolar distance. Increasing the direct current intensity of the cell leads to a decrease of the thickness of the lateral ledges of frozen electrolyte, and vice versa.

3. Changing the heat losses of the cell by changing the height of the metal on the bottom of the cell. De creasing the height of the metal leads to a decrease of the thickness of the lateral ledges of frozen electrolyte, and vice versa.

4. Changing the heat losses of the cell by changing the thermal insulation of the cell. The simpliest method to do this consists in changing the height of the alumi num oxide layer on the crust of frozen electrolyte above the liquid electrolyte and/or on the anodes. Increasing the height of the aluminum oxide layer leads to a decrease of the thickness of the lateral ledges of frozen electrolyte, and vice versa.

5. Changing the heat losses of the cell by changing the number of crust breaking operations per day. Decreasing this number leads to a decrease of the thickness of the lateral ledges of frozen electrolyte, and vice versa.

6. Changing the composition of the liquid electrolyte by changing the concentration of electrolyte additives such as AlF LiF, Cali, MgF NaCl.

Several of the measures mentioned above under (1 to (6) can be carried out in combination.

The advantage of the method according to my invention lies in the fact that disturbing changes of the thickness of the lateral ledges of frozen electrolyte can be avoided. Hereby a more uniform all performance is specific consumption of electrical energy.

what l claim is:

1'. A niethodfor controlling the thickness of the lateral ledges of frozen electrolyte in a cell for the recovery of aluminum metal by the electrolysis of aluminum oxide dissolved in an electrolyte including a fluoride melt, said electrolysis being. conducted with at least one anode connected to and movable with an anode beam operative for raising and lowering said anode with respect to said electrolyte, said method comprising the steps of:

a. measuring the instantaneous ohmic resistance of said cell at predetermined regular intervals of time during which measurements there are no anode effects which can substantially affect the measured instantaneous ohmic resistance and during which measurements there are no working operations carried out or have been previously carried out which can substantially affect the measured instantaneous ohmic resistance, measuring the average instantaneous ohmic cell resistance from a plurality of the measured instantaneous ohmic cell resistances obtained during a predetermined period of time, measuring the base resistance of the cell, obtaining the difference AR between the average instantaneous ohmic cell resistance and the base resistance;

b. moving said anode beam to minimize the difference between the measured instantaneous ohmic cell resistance and the base resistance when the difference AR exceeds a predetermined amplitude;

c. measuring the level of the anode beam;

d. thereafter repeating the steps a) to c) after a time interval not exceeding about one day;

e. measuring the differences AB between the level of the anode beam obtained in the steps c) and d) and compensating for level differences due to the tapping of and the accumulation of the aluminum metal in the cell; and

f. changing the thickness of the lateral ledges to a predetermined amplitude whenever the difference AB lies outside a predetermined range of values.

2. The method according to claim 1, in which there are a plurality of anodes having an interpolar distance and the interpolar distance is changed while maintaining a constant direct current intensity in the cell in order to change the thickness of the lateral ledges of frozen electrolyte.

3. The method according to claim 1, in which there are a plurality of anodes having an interpolar distance and the direct current intensity of the cell is changed while maintaining a constant interpolar distance in order to change the thickness of the lateral ledges of frozen electrolyte.

4. The method according to claim 1, in which the height of the aluminum metal on the bottom of the cell is changed in order to change the thickness of the lateral ledges of frozen electrolyte.

5. The method according to claim 1, in which the height of the aluminum oxide layer on the crust of fro- Zen electrolyte above the liquid electrolyte is changed in order to change the thickness of the lateral ledges of frozen electrolyte.

6. The method according to claim 1, in which there are a plurality of anodes and the aluminum oxide layer on the anodes is changed in order to change the thickness of the lateral ledges of frozen electrolyte.

7. The method according to claim 1, further comprising crust breaking operations and the number of crust electrolyte.

8. The method according to claim 1, in which the composition of the liquid electrolyte is changed in order to change the thickness of the lateral ledges of breaking operations per day is changed in order to frozen electrolyte.

change the thickness of the lateral ledges of frozen 

1. A METHOD FOR CONTROLLING THE THICKNESS OF THE LATERAL LEDGES OF FROZEN ELECTROLYTE IN A CELL FOR THE RECOVERY OF ALUMINUM MTAL BY THE ELECTROLYSIS OF ALUMINUM OXIDE DISSOLVED IN AN ELECTROLYTE INCLUDING A FLOURIDE MELT, SAID ELECTROLYSIS BEING CONDUCTED WITH AT LEAST ONE ANODE CONNECTED TO AND MOVABLE WITH AN ANODE BEAM OPERATIVE FOR RAISING AND LOWERING SAID ANODE WITH RESPECT TO SAID ELECTROLYTE, SAID METHOD COMPRISING THE STEPS OF: A MEASURING THE INSTANTANEOUS OHMIC RESISTANCE OF SAID CELL AT PREDETERMINED REGULAR INTERVALS OF TIME DURING WHICH MEASUREMENTS THERE ARE NO ANODE EFFECTS WHICH CAN SUBSTANTIALLY EFFECT THE MEASURED INSTANTANEOUS OHMIC RESISTANCE AND DURING WHICH MEASUREMENTS THERE ARE NO WORKING OPERATIONS CARRIED OUT OR HAVE BEEN PREVIOUSLY CARRIED OUT WHICH CAN SUBSTANTIALLY AFFECT THE MEASURED INSTANTANEOUS OHMIC RESISTANCE, MEASURING THE AVERAGE INSTANTANEOUS OHMIC CELL RESISTANCE FROM A PLURALITY OF THE MEASURED INSTANTANEOUS OHMIC CELL RESISTANCES OBTAINED DURING A PREDETERMINED PERIOD OF TIME, MEASURING THE BASE RESISTANCE OF THE CELL, OBTAINING THE DIFFERENCE $R BETWEEN THE AVERAGE INSTANTANEOUS OHMIC CELL RESISTANCE AND THE BASE RESISTANCE, B. MOVING SAID ANODE BEAM TO MINIMIZE THE DIFFERENCE BETWEEN THE MEASURED INSTANTANEOUS OHMIC CELL RESISTANCE AND THE BASE RESISTANCE WHEN THE DIFFERENCE R EXCEEDS A PREDETERMINED AMPLITUDE, C. MEASURING THE LEVEL OF THE ANODE BEAM, D. THEREAFTER REPATING THE STEPS A) TO C) AFTER A TIME INTERVAL NOT EXCEEDING ABOUT ONE DAY, E. MEASURING THE DIFFERENCES B BETWEEN THE LEVEL OF THE ANODE BEAM OBTAINED IN THE STEPS C) AND D) AND COMPENSATING FOR LEVEL DIFFERENCES DUE TO THE TAPPING OF AND THE ACCUMULATION OF THE ALUMINUM METAL IN THE CELL, AND F. CHANGING THE THICKNESS OF THE LATERAL LEDGES TO A PREDETERMINED AMPLITUDE WHENEVER THE DIFFERENCE $B LIES OUTSIDE A PREDETERMINED RANGE OF VALUES.
 2. The method according to claim 1, in which there are a plurality of anodes having an interpolar distance and the interpolar distance is changed while maintaininG a constant direct current intensity in the cell in order to change the thickness of the lateral ledges of frozen electrolyte.
 3. The method according to claim 1, in which there are a plurality of anodes having an interpolar distance and the direct current intensity of the cell is changed while maintaining a constant interpolar distance in order to change the thickness of the lateral ledges of frozen electrolyte.
 4. The method according to claim 1, in which the height of the aluminum metal on the bottom of the cell is changed in order to change the thickness of the lateral ledges of frozen electrolyte.
 5. The method according to claim 1, in which the height of the aluminum oxide layer on the crust of frozen electrolyte above the liquid electrolyte is changed in order to change the thickness of the lateral ledges of frozen electrolyte.
 6. The method according to claim 1, in which there are a plurality of anodes and the aluminum oxide layer on the anodes is changed in order to change the thickness of the lateral ledges of frozen electrolyte.
 7. The method according to claim 1, further comprising crust breaking operations and the number of crust breaking operations per day is changed in order to change the thickness of the lateral ledges of frozen electrolyte.
 8. The method according to claim 1, in which the composition of the liquid electrolyte is changed in order to change the thickness of the lateral ledges of frozen electrolyte. 